This invention relates generally to temperature compensation of field devices, and in particular to compensation utilizing a temperature-averaging sensor that characterizes an extended region of the field device.
The term “field device” refers to a broad range of process management devices designed to measure and control process parameters such as pressure, temperature and flow rate. Field devices include both transmitters, which are configured to measure or sense a process parameter with a sensor module, and controllers, which are configured to modify or control such a parameter with a control module (for example, by positioning a valve or regulating a pressure). Field devices also include multi-sensor transmitters such as pressure/temperature transmitters, and integrated controllers comprising both sensor modules and control modules (for example, integrated flow controllers). Field devices can also utilize more generalized field modules, which can incorporate a range of related measurement and control functions (as, for example, in an integrated hydrostatic tank gauge system).
Field devices have broad utility in applications that include manufacturing, fluid processing, food preparation and environmental control, and are applied to a wide range of process materials including air, water, liquid hydrocarbon fuels, natural gas, glues, resins, thin films, and thermoplastics such as polyvinyl chloride (PVC). Most of these applications require at least some form of temperature compensation, which in general must address both direct and indirect effects. Direct effects include temperature dependencies in the process material itself, particularly with respect to pressure and volume-related measurements. Indirect effects include temperature dependencies in the field device, such as thermoelectric sensor response, temperature dependencies in analog-to-digital (A/D) or digital-to-analog (D/A) converters, and other related effects.
Direct temperature compensation requires measurement of the process material, which often implicates large inventories and flow volumes. This is particularly true, for example, in energy-sector applications like petroleum refining and bulk fuel transportation, where process temperatures may vary substantially even within a single flow unit or storage volume. Direct temperature compensation therefore employs multi-spot temperature sensors, or, alternatively, temperature-averaging sensors that characterize an extended region of the process material.
Temperature compensation directed toward field devices, on the other hand, has traditionally relied upon single-spot compensation sensors. Field devices are generally small as compared to typical process volumes, and, in the idealized case, temperatures may not vary significantly on this scale. Moreover, because field devices emphasize simple, compact, and robust design methods, it can in any case be difficult for them to incorporate complex multi-spot compensation systems.
Nonetheless, under actual operating conditions significantly non-uniform temperature conditions do arise. Process heat flow, maintenance operations, and changing ambient conditions all produce temperature gradients, which can sometimes exceed 10-20° C. across a typical field device. Under such conditions a single-point sensor may not adequately characterize the field device, resulting in signal drift, bias, and other effects. Thus there remains a need for a temperature compensation technology that can overcome this deficiency, and so improve upon the prior art.